In recent years, for example, a few of SiGe-HBTs (hetero-bipolar transistors) including an SiGe base layer, are being used in high-frequency analog circuits, such as cellular telephones and wireless LANs. SiGe-Bi CMOS processing technology, etc., is used for the fabrication thereof. Japanese Unexamined Patent Publication Nos. 2001-68480, 2001-168105, and 2001-223222 disclose bipolar transistors, wherein improved operating speeds and lower power consumption are the main objects to be achieved. For example, Japanese Unexamined Patent Publication No. 2001-223222 proposes increasing the doping concentration of the emitter layer to reduce power loss by lowering the voltage drop between the emitter and collector when the transistor is turned on.
To reduce power consumption, in a semiconductor integrated circuit using a plurality of bipolar transistors, it is desirable that the ON-state voltage (the base voltage required to turn the transistor into an ON state) be independently set for each bipolar transistor depending on the circuit characteristics, etc. For example, power consumption can be reduced by setting the ON-state voltage low in a transistor on the output side of a semiconductor integrated circuit so that current may easily flow, and setting the ON-state voltage high in a transistor on the input side so that current cannot flow easily. However, it was difficult to separately set the ON-state voltage at desired levels without degrading the characteristics of each transistor.